The field of the present invention relates generally to sensing the velocity of a moving surface and, more particular, to a motion sensor to detect the passage of registration marks formed around the circumference of a photoreceptor belt in a xerographic printing apparatus to measure the speed of the belt.
In printing systems that utilize an elongate image receiving surface, such as a paper web or a belt, the receiving surface reaches a first marking station where a marking material of a first color is applied to the surface, e.g., by firing ink jets, exposing an image on a photoconductive material, or applying toner particles to a selectively imaged photoconductive member. The receiving surface then moves on to a second marking station, where an image or marking material of a second color is applied, and so forth, depending on the number of colors. The timing of the actuation of the second marking station is controlled as a function of the speed of the image receiving surface so that the images applied by the two marking stations are registered one on top of the other to form a composite, multicolor image. A high degree of process direction alignment can be achieved by knowing the speed or position of the image receiving surface. Currently the speed is measured with an encoder at a certain location and then the images are timed accordingly. For example, an encoder is associated with a drive nip roller. The rotational speed of the roller is used to calculate the speed of the image receiving surface passing through the nip. The time for actuating the first, second, and subsequent marking stations is then calculated, based on their respective distances from the drive nip roller and the determined speed of the image receiving surface.
In the case of an electrophotographic printer, an encoder may be placed on the photoreceptor belt to measure the exact speed of the belt at each instant of time. Additional techniques for determining photoreceptor speed include calculation based on belt module encoder frequency, encoder roll diameter, and photoreceptor belt thickness. The photoreceptor speed can then be used to time the firing of the laser raster output scanner (ROS) or light emitting diode (LED) bar so that an even spacing of lines is imaged on the photoreceptor. The surface speed calculation is also used for sensor timing, image sync generation, calculations for image on paper setup, and speed matching with the media path. While adequate for current printing process speeds, the current techniques would not be adequate for designs that need an increase in process speed. Because current speed calculations are based on nominal values, they tend to produce photoreceptor speed calculations with variability or tolerances that are not within an acceptable range.
For the reasons stated above, and for other reasons stated below which will become apparent to those skilled in the art upon reading and understanding the present specification, there is a need in the art for a more accurate measurement of photoreceptor speed.